1. Field of the Invention
The present invention relates to a method for joining heat exchanger tubes with headers, and more particularly to a method for joining a plurality of flat tubes with heaters in an heat exchanger.
2. Description of the Prior Art
In general, any type of vehicles is provided with an engine cooling system having a heat exchanger for cooling its engine during operation. In the engine cooling system of the vehicle, a cooling liquid such as cooling water is heated by the engine, and then supplied to the heat exchanger through conduits. In the heat exchanger, the thus heated cooling liquid gives its heat to open air through walls of the heat exchanger to permit the open air to cool the cooling liquid. The thus cooled cooling liquid is then returned to the engine through the conduits.
The heat exchanger or radiator is generally constructed of: a pair of cooling liquid tanks spaced apart from each other; and a plurality of tubes interposed therebetween and arrange parallel to each other. More particularly, in the heat exchanger, a pair of headers spaced apart from each other are employed. Each of the headers constitutes a part of each of the tanks, and is connected to each of opposite ends of the tubes which are interposed between the tanks and arranged parallel to each other therebetween. In construction of the tank, the tank is, for example, assembled from two parts one of which is the header and the other of which is a tank body assuming a pan-shaped form. In this case, a flange is formed in an opening portion of the tank body, and a peripheral portion of the header is hermetically connected to the flange of the tank body. On the other hand, the header is provided with a plurality of holes each of which an end portion of each of the heat exchanger tubes passes through to join the tubes and the header, and the tubes are hermetically connected to the header at joints therebetween. In case that the heat exchanger or radiator is of a tube-and-fin type, such heat exchanger consists of the tubes and a plurality of air fins placed around the tubes to improve heat transfer. In operation of the engine, a hot cooling water flows out of water jackets of the engine, and enter one of the tanks of the heat exchanger. Then, the cooling water flows to the tubes through which the cooling water enters the other of the tanks of the heat exchanger. In the above process, the hot cooling water is cooled by open air while passed through the tubes. The cooling water having been cooled and received in the other of the tanks returns to the water jackets of the engine. In some heat exchangers with the above construction, the cooling water leaks from the joints between the tubes and the headers. In general, outer peripheral surfaces of the tubes and peripheral portions of the holes of the headers adjacent to the outer peripheral surfaces of the tubes are brazed or soldered to each other to constitute liquid-tight joints between the tubes and the headers. In service life, the heat exchanger is heated during use of the engine while cooled during non-use of the engine. Namely, the heat exchanger is subjected to heat cycles in its service life to suffer from thermal stresses caused by differences in thermal expansion and contraction between materials of the tubes and the headers.
Such thermal stresses appearing in the materials of the tubes and the headers are concentrated on the joints, and often cause fatigue failures of the joints.
Heavy construction machines generally employ large-sized heat exchangers for cooling their engines in operation. Such large-sized heat exchangers are very heavy since they contain ample amounts of cooling water. The large-sized heat exchanger employs flat tubes as its constituent element, which flat tubes are supported by metal plates or headers each of which is made of brass and has a thickness of about 3 to about 7 mm. Thermal stresses appearing in joints between the headers and the flat tubes of the large-sized heat exchanger are considerably large in comparison with those appearing in ordinary-sized heat exchangers. These thermal stresses are produced by thermal expansion and contraction of the materials of the heat exchanger. In addition to the thermal stresses, the large-sized heat exchanger suffers from mechanical stresses resulted from its heavy weight. In order to withstand these thermal stresses and mechanical stresses, the joints between the tubes and the headers must have sufficient strength.
In the heat exchanger provided with the flat tubes, the joints between the flat tubes and the headers is increased in strength according to a method for increasing strength of joints between headers and flat tubes, which method is disclosed in Japanese Patent Laid-Open No. Hei 1-281399. According to the method disclosed in the Japanese Patent Laid-Open No. Hei 1-281399: end portions of the flat tubes are passed through elliptical holes formed in the headers; opposite minor axis side walls of each of the end portions of the flat tubes are cut by a cutter and bent radially outwardly at right angles; and the thus radially outwardly bent wall portions of the flat tubes are brazed or soldered to the headers to form the joints therebetween. However, in this method, since only the minor axis side walls of the flat tubes are bent, four notches are inevitably formed in each end wall portion of each of the flat tubes, which notches appear at positions between the thus cut minor axis walls and major axis walls separated from such minor axis walls of the flat tubes. Consequently, axial loads acting on the flat tubes act in turn to the joints thereof to produce concentrated stresses at the notches of the joints. Therefore, the above conventional method fails to provide the joints with sufficient strength. In addition, the joints formed according to the conventional method produce turbulent flow of the cooling water in operation to considerably increase flow passage resistance of the heat exchanger. These are problems inherent in the conventional method.